Recently, the substitution of glass glazing with transparent materials which do not shatter or are more resistant to shattering than glass, has become widespread. For example, transparent glazing made from synthetic organic polymers is now utilized in public transportation vehicles, such as trains, buses, taxis and airplanes. Lenses, such as for eye glasses and other optical instruments, as well as glazing for large buildings, also employ shatter-resistant transparent plastics. The lighter weight of these plastics in comparison to glass is a further advantage, especially in the transportation industry where the weight of the vehicle is a major factor in its fuel economy.
While transparent plastics provide the major advantage of being more resistant to shattering than glass, a serious drawback lies in the ease with which these plastics mar and scratch, due to everyday contact with abrasives, such as dust, cleaning equipment and ordinary weathering. Continuous scratching and marring results in impaired visibility and poor aesthetics, and oftentimes requires replacement of the glazing or lens or the like.
One of the most promising and widely used transparent plastics for glazing is polycarbonate, such as LEXAN.RTM. resin, sold by General Electric Company. It is a tough material, having high impact strength, high heat deflection temperature, good dimensional stability, as well as being self-extinguishing, and is easily fabricated. Acrylics, such as polymethylmethacrylates, like PLEXIGLAS.RTM. resin, are also commonly and widely used glazing and lens materials.
Attempts have been made to improve the abrasion resistance of these transparent plastics. For example, scratch resistant coatings formed from mixtures of silica, such as colloidal silica or silica gel, and hydrolyzable silanes in a hydrolysis medium, such as alcohol and water, are known. Misch et al., U.S. Pat. No. 3,708,225; Clark, 3,986,997, 3,976,497 and 4,027,073; Armbruster et al., 4,159,206; and Ubersax, 4,177,315, for example, describe such compositions. Improved such compositions are also described in commonly-assigned copending U.S. Application Ser. No. 964,910, filed Nov. 30, 1978, and in U.S. Pat. No. 4,277,287 (Frye) on Nov. 30, 1978.
It has been discovered that such polysilicic acid coatings, especially if acidic, fail to adhere to certain plastic substrates, such as polycarbonate, and, even if prepared on the basic side of neutrality, they may adhere initially, but they will peel after brief light aging. In U.S. Pat. No. 4,299,746 (Frye), the addition of an ultraviolet (uv) light screening agent, such as 2,4-dihydroxy-benzophenone, is suggested, but in some cases this may have a plasticizing effect and polycarbonate seems to have a tendency to reject the coating on severe exposure. Another approach is to use a two-coat system in which, for example, an acrylic primer is first laid down, and this adheres to both the silicone and the polycarbonate resins and also serves as a binder for high levels of UV screens. The second coat is put on over the primer coat. See, for example, commonly assigned Humphrey, Jr., U.S. Pat. No. 4,188,451, and Application Ser. No. 964,911, filed Nov. 30, 1978. To avoid the need for a primer, commonly assigned copending Application Ser. No. 34,164, filed Apr. 27, 1979, suggests replacing the usual solvents, e.g., isobutanol, with a more aggressive solvent, e.g., an ester, a ketone, a nitroparaffin, or the like. However, these are expensive and generally might require process modifications.
The use of modified organic UV screens in related compositions is also known from Proskow U.S. patent 4,051,161. Proskow describes the use of certain reaction products of an organic UV stabilizer and an epoxy silane. The reaction products have divalent connecting aliphatic secondary alcohol radicals between the silicon atom and the organic stabilizing group which are inherently oxidatively unstable. These reaction products of Proskow are utilized in a mixture of polysilicic acid and a haloethylene-hydroxy vinylether copolymer.
Alkoxy silanes useful as UV screens having organic UV stabilizing groups attached to silicon through divalent aliphatic connecting groups free of such oxidatively unstable secondary alcohol functionality are also shown in commonly assigned U.S. Pat. No. 4,278,804; Ashby, U.S. Pat. No. 4,321,400; U.S. Pat. Nos. Ching, 4,307,240; Ching, 4,316,033; Ching Ser. No. 154,624, filed concurrently herewith. All of the above-mentioned patents, as well as the application, are incorporated herein by reference.
It has now been discovered that aqueous cohydrolyzates of polysilicic acid and such alkoxy silanes can be directly applied onto a thermoplastic substrate as a silicon hardcoat without the use of primer, or haloethylene-hydroxy vinyl ether copolymer adhesive promoter to produce composite articles having superior photostability and abrasion resistance. Accordingly, aqueous cohydrolyzates made in accordance with the practice of the present invention, as defined hereinafter, can be applied directly onto an unprimed thermoplastic substrate without an additional adhesive such as a haloethylene-hydroxy vinyl ether copolymer. By way of illustration, hydrolysis of 0.8 to 4 parts (or 4-20% based on solids) of the alkoxysilane-functionalized UV screens, such as (.gamma.-triethoxysilane)-propyl-3,3-diphenyl-2-cyanoacrylamide, 2-hydroxy-3-(.gamma.-triethoxy silane)propyl-5-methylphenyl benzotriazole, and the like, with 100 parts of a polysilic acid hardcoat solution for 10 days, e.g., at pH 6.8-7.8, results in novel UV screen-containing silicone hardcoats. These improved silicone coatings are adapted for use directly on plastics, e.g., polycarbonates and acrylics, without a primer.